Methacrylic resins typified by polymethyl methacrylate (PMMA) are characterized, as transparent resins, by having higher optical transmittance, weather resistance, and rigidity than those of other plastic transparent resins and have heretofore been widely used for purposes such as components for vehicles, lighting equipment, materials for building construction, signboards, nameplates, paintings, and windows for display apparatuses.
The methacrylic resins are often used, particularly, as materials for lens members in vehicle lamps.
The vehicle lamps are usually constituted by a lens portion welded to a housing portion. Hot plate welding has typically been practiced as a conventional method for welding a lens molded article for use in the lens portion and a housing molded article.
The hot plate welding, however, presents problems such as poor appearance or poor looks of welded surface caused by resin threads after heating. Therefore, laser welding, which is free from such problems, has been increasingly adopted in recent years.
On the other hand, for use in vehicles, thinning has proceeded recently for the purpose of reducing weights in association with improvement in fuel economy, while upsizing has proceeded for realizing fresh designs. There arises a new problem that warpage occurs in molded articles due to charging pressure increased during the charging of resin materials into molded article materials, resulting in reduction in welding strength or solvent resistance. Furthermore, there also arises a deformation problem ascribable to heat.
Patent Literature 1 discloses a resin improved in terms of flowability in injection molded article and solvent resistance.
Patent Literature 2 discloses a technique using a methacrylic resin in lens members for marker lights for vehicles.
Patent Literature 3 discloses a method which involves applying pressure to a warped molded article and laser-welding a lens molded article and a housing molded article kept in close contact.